Fill it with awesome.

It depends, what are you doing with the car? TSD rallies? Stick an Alfa or something in there. Is this for performance rallying? Most people stick gauges or control switches.

If I was bored and had to make a project out of it, I'd try and build a touchscreen car-puter that has a USB GPS connected or find a wheel probe that can be logged with a PC. Then, see if you can find a rally odo type app and use that for TSD events. When you're not using it, it could do music, or maybe you could try and get an ODB2 connection to it and have live datalogging right there in the dash.

As for the nav unit itself, try and sell it on EBay?